When drilling a borehole through an earthen formation a pilot hole is drilled by a pilot bit and the hole can be enlarged by an under reamer. Under reamers have arms with cutters thereon that cut into the formation to enlarge the borehole to its intended gauge.
Under reamers are useful in casing drilling, wherein the pilot bit must be of a size to pass through the bore of the casing and is, therefore, not sized to drill a borehole of a gauge that the casing can pass therethrough. Therefore, the pilot bit drills the pilot hole into the formation and under reamers enlarge the hole behind the pilot bit to a gauge greater than the casing outer diameter to permit advancement of the casing into the borehole. Under reamers are also useful when extending a borehole below installed casing. In such embodiments, the under reamer arms are collapsible to permit the under reamer to be moved through the bore of the casing and are expandable downhole to permit drilling of a borehole to a gauge greater than the outer diameter of the casing.